Voltage multiplying circuits use either capacitive or inductive phenomena to step up an input voltage. Even the recently introduced piezoelectric step-up transformers use piezo-capacitive phenomena to generate stepped up output voltage. Inductive and piezoelectric voltage multipliers are for size and weight reasons not practical for high voltage generation in handheld terminals. For use with integrated circuitry capacitive mulipliers are the preferred choice.
Basically two types of on-chip circuits are known that generate pulses higher than the supply voltage: diode chain multiplier and parallel/serial switching of capacitors.
The parallel/serial capacitive multiplying circuits are principally based on the Ervin Marx multiplier, where capacitors are charged in parallell and discharged in series. Although widely used in high voltage high power pulse applications, this kind of store and stack system has drawbacks, especially when a steady level high voltage is needed, because the stack will discharge and the output voltage will drop after the initial pulse. In the original form the switching was done with the help of sparkgaps which all will all break down substantially simultaneously because the voltage will increase over the remaining sparkgaps when the first sparkgap triggers or is triggered. Instead of sparkgaps, for lower voltages controlled switches can be used instead. An example of an improved variant of the Marx multiplier called the Mosmarx multiplier was given by P. E. K. Donaldsen: “The Mosmarx voltage multiplier”, Electronics & Wireless World, August 1988, pages 748-750. Here metal oxide semiconductors (MOS) switches were used instead of spark gaps. When continuous output is needed high voltage charge is stored in a separate reservoir capacitor, isolated by a serial diode from the output stage, and the switches are operated continuously. The continuous operation prevents the use of micromechanical (MEMS) switches which have a limited lifetime and/or operating frequency.
Most high voltage generators are typically feeding into a reservoir capacitor. When a high voltage pulse is created, it charges the capacitor with energy taken from the supply. When the high voltage pulse is to be terminated, the charge on the output capacitor is normally discharged to ground on the falling edge of the pulse and all the energy in the capacitor is lost. This loss is proportional to the reservoir capacitance, but too low a capacitance will cause output voltage ripple. For pulsed use it would however be energy efficient if the output capacitance would be small and mainly consist for example of the parasitic capacitance of a display device.
The other common voltage multiplying circuit is based on cascaded diode pumps. Such multipliers are based on chained voltage multipliers, using the simple two diode D1, D2 voltage doubling stage coupled to the driving voltage waveform input U1 via a capacitor C1 illustrated in FIG. 2. In this doubling stage the output voltage U2 over a capacitor C2 is twice the input voltage U1 for a rectangular driving pulse and 2.82 times the RMS value of a sinusoidal U1 driving waveform, if the voltage drop over the diodes is disregarded.
When cascaded, such simple voltage multipliers will form the well known Cockcroft-Walton multiplier chain, popularized by John Cockcroft and Ernest Walton, and illustrated in FIG. 3, wherein the chain consists of capacitors C1 to C8 and diodes in a known manner.
In order to increase the efficiency and to reduce the output ripple the multiplier according to FIG. 3 can be doubled to form a push pull multiplier, illustrated in FIG. 4. This multiplier can be considered as two separate Cockcroft-Walton multiplier diode chains provided with diodes and capacitors C6 to C8 and C61 to C81, charging a common reservoir capacitor chain, C2, C3, C4 and C5, and the two voltage multiplier chains are each driven by respective phase shifted inputs U11 and U12 via capacitors C1a, C1b. Each cascaded diode multiplier chain need only supply half the current and the output ripple will be reduced because of the increased frequency of the charging action even if the reservoir capacitor chain C2, C3, C4 and C5 is of the same size as in FIG. 3. In the same way other multi phase multipliers can be envisaged.
FIG. 5 shows the same multiplier as FIG. 4, with the phased input signals U11 and U12 via capacitors C1a and C1b and the output to U2 through the diodes D7a and D7b and smoothed by C5, which is at the top of the reservoir capacitor chain. FIG. 5 can be simplified to a also previously known split type multiplier according to FIG. 6 by removing the reservoir capacitor chain and connecting the anode of each step-up diode directly to the cathode of the next stage step-up diode. Because two serially connected diodes are now replaced with a single diode this can be done with little or no efficiency loss. Another advantage of the circuit of FIG. 6 that can be used to advantage in the present invention is the capacitor free output, because U2 is fed through the diodes D7a and D7b without a reservoir capacitor. This is more clearly illustrated in by FIG. 7, which is actually FIG. 6 redrawn in a more common form.
FIG. 8 illustrates how a multiplier can be connected to boost a supply voltage Udc. The output voltage will be increased by the same amount as the supply voltage itself. The previous examples in FIG. 2 to FIG. 7 were drawn as ground referenced, but could use this method as well.
FIG. 9 illustrates how the a voltage multiplier diode chain can be arranged by driving the capacitors in parallell by U11 respectively U12. To drive the capacitors in parallel is more efficient than driving them through previous stages. This is a efficient and parts count reducing method that can be used whenever the generated voltages are within the capacity of the circuitry.
FIGS. 2 to 9 have illustrated various known multipliers using diode chains as switches. The earlier mentioned series/parallel Marx multiplier uses switches and not diodes, and a typical Marx multiplier is depicted in FIGS. 10a and 10b, where FIG. 10a shows the charging in parallel of C1, C2, C3 and C4 from U1 using the even numbered switches S2, S4, S6, S8, S10 and S12 and FIG. 10b shows the high voltage output phase when the even numbered switches are opened and the odd numbered switches S1, S3 and S5 are engaged to connect the charged capacitors in series in order to generate the high output voltage U2. Any outside load capacitance will be connected in parallel with C1, C2, C3 and C4 in the charging phase. The diode D101 prevents the generated high voltage 32 to be leaked back into the supply 31 during the high voltage output pulse and does the same also in the charging phase if the capacitor voltage because of charge charing is higher than the supply voltage.
In many applications a high-voltage source is needed that can be switched on and off rapidly without loosing energy and without the use of a high-voltage output switch, which is difficult to implement.
Using a serial/parallel multiplier with a reservoir capacitor output would need such high-voltage switch or else the reservoir charge would need to be bled-off every time. Charging the reservoir capacitor and then bleeding off the charge when there is no need for high voltage decreases efficiency.
Using a serial/parallel multiplier without a diode fed reservoir capacitor will however limit the length of a high-voltage pulse in single pulse use and in continuous use it will result in excessive ripple.
A diode chain multiplier will also need a high voltage output switch in order to preserve efficiency when high voltage is needed intermittently, like for example in handheld terminals. In handheld terminals, where efficiency is of paramount concern, the display is using high voltage only when active, and is typically switched on and off intermittently.